One of the most robust measures of Earth’s changing climate is that winter is warming faster than other seasons. The cascade of changes this brings, including ice storms and rain in areas that were once reliably below freezing, are symptoms of what I call “warming winter syndrome.”
Winter warming represents the global accumulation of heat. During winter, direct heat from the sun is weak, but storms and shifts in the jet stream bring warm air up from more southern latitudes into the northern US and Canada. As global temperatures and oceans warm, that stored heat affects both temperature and precipitation.
The warming is evident in changes in growing seasons, reflected in recent updates to plant hardiness zones printed on the back of seed packets. These maps show the northward and sometimes westward movement of freezing temperatures in eastern North America.
The shift in this freezing line between snow and rain could mean ice storms in places and at times when communities are unprepared to handle them, as several parts of the US saw in early 2024.
Ice storms and sleet
I study the impact of global warming and have documented changes in climate and weather over the past decades.
On average, freezing temperatures move north and, along the Atlantic coast, into the interior of the continent. For individual storms, the transition to freezing temperatures, even in the dead of winter, can now occur as far north as Lake Superior and southern Canada, in places where 50 years ago it was reliably below freezing from early December through February lay.
When the temperature is close to freezing, the water can be rain, snow or ice. Regions on the colder side, which have historically been below freezing and snowy, are seeing an increase in ice storms.
The character of snow also changes around the frost line. When the temperature is well below freezing, the snow is dry and fluffy. Around freezing, the snow has large, wet, heavy flakes that turn roads to mud and stick to tree branches and bring down power lines.
Because the climate in which snowstorms form is warmer due to the global accumulation of heat, and wetter due to more evaporation and warmer air that can hold more moisture, individual snowstorms can also result in more intense snowfall. However, as temperatures warm in the future, the scales will tilt toward rain and the total amount of snow will decrease.
On the warmer side of the freezing line, winter rain is already becoming the dominant form of precipitation, a trend that is expected to continue. With warmer oceans an important source of moisture, the already wet eastern US can expect more winter precipitation over the next thirty years. Looking into the future, soggy, wet winters are more likely.
Disaster and water planning are becoming more difficult
For communities, planning for water supplies and extreme weather becomes more complicated in a rapidly changing climate. Planners cannot count on the weather thirty years in the future to be the same as the weather today. It’s changing too quickly.
In many places the snow will not last until late spring. In regions like California and the Rockies that rely on snow cover for water year-round, that supply will become less reliable.
Rain falling on snowpack can also accelerate melting, cause flooding and change the flow of creeks and rivers. This is reflected in changing discharge patterns in the Great Lakes, and led to flooding on the East Coast in January 2024.
For road planners, the number of freeze-thaw cycles that can damage roads will increase during the winter in many regions unaccustomed to such rapid shifts.
A particularly interesting effect occurs in the Great Lakes. The Great Lakes are already not freezing as early or as completely as in the past. This has major consequences for the famous lake effect precipitation zones.
Because the lakes are not frozen, more water evaporates into the atmosphere. In places where air temperatures are still below freezing in winter, lake effect snow increases. The Buffalo, New York, region received more than six feet of snow in 2022 due to a lake-effect storm. Because air temperatures flirt with the freezing line, these events are more likely to be rain and ice than snow.
These changes don’t mean the cold is gone for good. There will be instances where Arctic air flows into the US. This can cause sudden freezes and fog as warm, wet air flows back over the frozen surface.
Huge consequences for economies
What we experience in warming winter syndrome is a consistent and robust set of symptoms on a feverish planet.
November and December will be milder; February and March will be more like spring. Winter weather will become more concentrated around January. There will be unknown variability with snow, ice and rain. Some people will say these changes are great; less snow needs to be shoveled and heating costs have fallen.
But on the other hand, entire economies are ready for winter, many crops depend on cool winter temperatures and many farmers rely on freezing weather to keep pests in check. Every time there are changes in temperature and water, the conditions in which plants and animals thrive change.
These changes, which affect outdoor sports and recreation, commercial fishing and agriculture, have enormous consequences not only for ecosystems, but also for our relationship with them. In some cases, traditions will be lost, such as ice fishing. In general, people will have to adapt almost everywhere.
This article is republished from The Conversation, a nonprofit, independent news organization providing facts and analysis to help you understand our complex world.
It was written by: Richard B. (Ricky) Rood, University of Michigan.
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Richard B. (Ricky) Rood receives funding from the National Oceanographic and Atmospheric Administration (NOAA).